The invention relates to a capped high-pressure discharge lamp comprising
discharge vessel having a first and a second neck-shaped portions facing one another and having seals through which respective first and second current supply conductors are passed to a pair of electrodes positioned in the discharge vessel,
a metal clamping member which clamps around the first neck-shaped portion of the discharge vessel,
a metal fixation member provided with first tongues which are connected to the clamping member and with second tongues having free ends,
a lamp cap of insulating material connected to the discharge vessel, which lamp cap at a side facing the discharge vessel has a first, circumferential cavity in which the fixation member is secured and a second, central cavity in which the first neck-shaped portion is accommodated, which central cavity affords access to a side of the lamp cap facing away from the discharge vessel, where the lamp cap carries a first and a second contact member to which the first and second current supply conductors, respectively, are connected.
Such a lamp is known from EP-0 309 041 A, which corresponds to U.S. Pat. No. 4,868,456, and is designed for use as a vehicle headlamp.
In order to be able to ignite such a lamp while it is still hot, use is made not only of a high voltage of several kV, but also of a high frequency of several kHz.
The lamp renders it possible to use a lantern, or reflector, of very small height, for example 5 cm, owing to its high brightness. In order to be accommodated therein, however, the lamp must be of compact construction.
In the known lamp, the clamping member is connected to the fixation member by means of a bush. The clamping member, the bush and the fixation member are necessary in that case for fixing the discharge vessel in the lamp cap in a position in which the pair of electrodes is aligned relative to reference locations at the lamp cap. During alignment the discharge vessel can be shifted, tilted, and rotated, as required.
Constructional parts of the lamp are under electrical tension and even pass current. The fixation member is included in the current circuit through the lamp, and the bush and the clamping member are connected to it.
It has been found that the compact construction of the lamp and the high applied voltages with their high frequencies can cause harmful and disadvantageous electrical effects, such as corona discharges, owing to which the voltage across the lamp can drop and lamp materials can be attacked. Breakdown may take place in the course of time as a result. These effects do not occur at low frequencies, for example below 100 Hz, under otherwise equal circumstances as regards to voltage and geometry.